A lithographic printing plate generally comprises a lipophilic image area which receives an ink and a hydrophilic non-image area which receives fountain solution during printing. As such a lithographic printing plate precursor, a PS plate (presensitized plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer has so far been widely used, and the plate-making method generally comprises performing mask exposure through a lith film and then dissolving and removing a non-image area with a developing solution to thereby obtain a printing plate.
Digitized techniques of electronically processing image data with a computer have prevailed in recent years, and various image output systems corresponding to these digitized techniques have been put to practical use. With such a trend, a computer-to-plate technique of directly making a printing plate by scanning digitized image data with highly directive actinic radiation such as laser beams without using a lith film has been earnestly desired, and it has become an important technical problem to obtain a printing plate precursor well adapted to this purpose.
On the other hand, the plate-making process of the PS plate hitherto in use is indispensably accompanied by wet process of dissolution and removal of a non-image area after exposure, this is another problem which has been desired to be improved. In particular, global environmental protection has been a matter of concern in the industry at large in recent years. There are hence increased demands for simplification of processing, switching over to dry process, and no processing from the viewpoint of environmental aspect and rationalization of the process with digitization.
As one plate-making method which does away with former processing steps, there is a development on machine system of using a photosensitive layer capable of removing the non-image area of a printing plate precursor in usual printing process without carrying out former development process, and effecting development after exposure on a printing machine to thereby obtain a final printing plate. However, one big problem of the development on machine system is that the printing plate precursor must be stored under a completely light-shielded state and/or under a constant temperature condition after exposure, e.g., during the period of time until the printing plate precursor is mounted on a printing machine, because the fixation of the photosensitive layer is not performed.
On the other hand, solid state lasers having high output, e.g., a semiconductor laser and a YAG laser are inexpensively available in recent years. As a result, a method of using these lasers is promising as a plate-making method by scanning exposure. In the method of high power density exposure using these high output lasers, various development systems can be utilized differing from photo-reactions used in photosensitive materials for low to middle power density exposure. Light energy absorbed by photosensitive materials is converted to heat and desired development is caused by the generated heat.
A big advantage of a plate-making method utilizing heat mode recording is that the material is not sensitized by exposure to light of general illuminance level and in normal atmospheric temperature, and fixation of the image after exposure is not essential. Accordingly, for example, when a photosensitive layer which is insolubilized or solubilized by heat mode exposure is used in a plate-making process by the on-press development system, it becomes possible to realize a system in which the image obtained is not influenced even development (removal of a non-image area) is performed after the printing plate precursor is exposed to atmospheric light for a certain period of time after image exposure.
Accordingly, if heat mode recording is utilized, it will be possible to obtain a lithographic printing plate precursor which is adapted to the on-press development system.
A method is suggested as one preferred plate-making method of a lithographic printing plate based on heat mode recording, which comprises the steps of providing a hydrophobic image-forming layer on a hydrophilic support, imagewise exposing the image-forming layer by heat mode exposure to convert the solubility/dispersibility of the hydrophobic image-forming layer, and removing a non-image area by wet development, according to necessity.
However, the image-forming layer as above is not sufficient in heat sensitivity, hence the sensitivity to heat mode scanning exposure is extremely unsatisfactory. Further, it is also a problem in practical use that the discrimination of hydrophobicity/hydrophilicity before and after exposure, i.e., the change in solubility, is small. It is almost impossible to perform plate-making by the on-press development system with poor discrimination.